Process for surface protection of parts subject to high thermal stress



United States Patent 2,987,805 PROCESS FOR SURFACE PROTECTION OF PARTSSUBJECT TO HIGH THERMAL STRESS Gerhard Kubera and Gisela Prasse,Blnmberg, Baden,

Germany, assignors to Alfred Teves Maschinen- Armaturenfahrik KG.,Frankfurt am Main, Germany No Drawing. Filed May 24, 1957, Ser. No.661,276 Claims priority, application Germany May 26, 1956 1 Claim. (Cl.29-156.7)

The present invention relates to the surface protection of parts subjectto high thermal stress and corrosion, such as valve cones of intake andexhaust valves of combustion engines, turbine blades of jet units, andsimilar parts exposed to high temperatures and the incidence ofcorrosion.

It has been found that aluminum coatings are an effective protectionagainst corrosion and scale formation. Thus, it became widely known tocoat the exposed surfaces of intake and exhaust valve cones withaluminum in order to extend their useful life.

Aluminum coating is effected by dipping the parts into an aluminum meltor by spraying them with aluminum. Subsequent heat treatment in saltbaths results in a diffusion of the aluminum to the marginal zone of thebase material. An iron-aluminum alloy is formed thereby at the surfacewhich has high corrosion resistance.

However, the known methods have certain disadvantages. Thus, during thediffusion-heating the accurately finished parts are somewhat distorted,and since in most cases an after-machining is not possible, a largeamount of Waste products will result. The applicability of this methodis restricted thereby to few cases only.

It is the object of the present invention to overcome the abovementioned shortcomings of known coating methods and to provide a processfor surface treatment of parts subject to high thermal stress which issimple, dependable and permits to obtain the desired coated parts with aminimum quantity of waste to be discarded.

According to the invention this is accomplished by the followingoperational steps: Parts subject to high thermal stress, e.g. intake andexhaust valves of combustion engines, combustion chambers. turbineblades, and the like. are first preformed to a part somewhat thickerthan the final shape and coated with aluminum in a manner known per se.Subsequently the parts are heated in an oxygen atmosphere to forgingtemperature or higher temperature so that a diffusion of aluminum willoccur into the base metal as well as oxidation of aluminum at thesurface, according to the equation 4Al+3O =2Al,O,. Thus, ceramicmaterial is formed on the surface which, as generally known, isresistant to highest temperatures. After this procedure the parts areforged or pressed to final shape. By that operation, the highlyheat-resistant oxides are forged into the base metal consisting ofaluminumiron so that they cannot become detached later.

Patented June 13, 1961 Articles treated in this manner will have a muchhigher heat and corrosion resistance than those produced by conventionalmethods due to the contents in ceramic components forming part of thesurface.

In the following, the method according to the invention will bedescribed by way of example with reference to a valve cone, but itshould be understood, that this is only given by way of illustration andnot of limitation and that many changes in the details can be madewithout departing from the spirit of the invention.

Example A valve cone of an exhaust valve of a combustion engine made ofknown valve steel f.i. silcrom 1, is first extruded. Then the aluminumis sprayed on the head of the valve cone in a known method.

In the next operation the valve cone is heated to a temperature of 960C. in an atmosphere of oxygen. This will cause a diffusion of thealuminum into the base metal to take place, while simultaneouslyoxidation at the surface will set in. This treatment is carried out overa period of 3 hours and results in the formation of A1 0 i.e. a highlyheat resistant ceramic material. In a final step the articles are forgedinto final shape. This operation results in the secure union of theceramic material with the alloy metal and in the formation of an excellent protective coating.

What we claim is:

A process for eifecting surface protection of valve cones made offerrous metals and subject to high thermal stress, by applying to thesurface of said valve cones a ceramic material, resistant to hightemperatures, which comprises coating said cones with molten metallicaluminum, heating the cones thus coated at approximately 960 C. in anatmosphere of oxygen for approximately 3 hours to allow diffusion of thealuminum into the ferrous metal to occur, while oxidation takes place atthe surface of the aluminum-coated cones with formation of highlyheat-resistant aluminum oxide, then heating said cones to forgingtemperature and forging them into the final shape.

References Cited in the file of this patent UNITED STATES PATENTS1,335,024 Peschko Mar. 30, 1920 1,409,017 Ortiz Mar. 7, 1922 1,823,869Baur Sept. 15, 1931 1,866,145 Wilson July 5, 1932 2,167,701 WhitfieldAug. 1, 1939 2,303,869 Quinlan et al. Dec. 1, 1942 2,573,229 Stern Oct.30, 1951 2,845,365 Harris July 29, 1958 FOREIGN PATENTS 731,281 GreatBritain June 8, 1955

